I'm currently reading The Real Planet of the Apes (2015), a popular science book on primate evolution. The author David R. Begun mentions how his graduate student, Amber MacKenzie, discovered a correlation between tail length and forelimb mobility (brief write ups of her respective 2013 and 2014 conference talks can be seen here and here). Basically, the shorter the tail, the more agile the forearm. Monkeys walk palmigrade on the tops of tree limbs. They have hinged wrists with both the radius and ulna connected to the hand bones. This morphology does not allow for much twisting motion. They rely heavily on their tails for shifting their center of gravity and (in some species) for grabbing hold of limbs like a fifth appendage. Therefore, monkeys with shorter tails would have to rely far more on agile wrists and grip strength for moving through trees. This has obvious implications for tail loss in hominoids.

Early fossil apes, such as Proconsul and its close cousin Ekembo (roughly 20 MYA), have the body of a monkey minus a tail (fig. 1 and 2). This includes a palmigrade posture, arms and legs of roughly equal length, and a long torso. Despite this morphology, they have powerful grasping hands and feet in comparison to monkeys.

Fig. 1 - (Left) A monkey skeleton. Fig. 2 - (Right) A reconstruction of Proconsul. A climbing example can be seen here.

This then brings us to a chicken or the egg-type question: Which came first, the agile forelimbs and hand strength or the shrunken tail? If I had to make an educated guess, I would think the disappearing tail selected for greater forelimb mobility and power. I believe terrestriality, or living on the ground, played a large role in this. Many of the short-tailed species that I am aware of live almost exclusively on the ground, such as Japanese Macaques and Mandrills (fig. 3 and 4). Spending time away from trees would make the tail a nearly useless appendage.* A primate wouldn’t need it for shifting the center of gravity or bracing. Then, when climbing into the trees during the day for energy rich food (like fruit or prey) or at night for sleeping, a flexible wrist and strong hands would be more beneficial.

I unfortunately don’t know enough about the forelimb morphology of these species to make any type of positive contribution to the subject. I plan to contact Begun and MacKenzie to see if any further insights have been made. In closing, I want to say that this information has certainly stoked the fires of my curiosity. The loss of tails in apes has always been something glossed over in much of the literature I’ve read.

* The only species that doesn’t appear to conform to this is the Baboon. To my knowledge, they are the most terrestrial of the Old World monkeys. But their tail still serves the purpose of signaling. This then might explain why it continues to be selected for.